The invention relates to an optical component containing an optically anisotropic layer, which latter has at least two regions with different molecular orientations. The anisotropic layer may, for example, be a retarder layer formed by cross-linked liquid-crystal monomers.
A particular use of the components according to the invention is in the field of protection against forgery and copying.
The demand for safeguarding banknotes, credit cards, securities, identity cards and the like against forgery is increasing constantly on account of the high-quality copying techniques which are available. Furthermore, in low-wage countries, imitations of branded products and copies of copyright-protected products, for example compact discs, computer software, electronics chips, etc. have been produced and exported worldwide. Because of the increasing number of forgeries, there is therefore a great need for new elements which are safeguarded against forgery and can be identified both visually and by machine.
In the field of copy-protecting banknotes, credit cards etc, there are already a considerable number of authentication elements. Depending on the value of the document to be protected, very simple or relatively highly complex elements arc employed. Some countries are content to provide banknotes with metal strips which come out black on a photocopy. Although this prevents them from being copied, elements of this type are very easy to imitate. In contrast to this, there are also more complex authentication elements, for example holograms and cinegrams. Authentication elements of this type are based on the diffraction of light by gratings and need to be observed under different viewing angles in order to verify their authenticity. These diffracted elements produce three-dimensional images, colour variations or kinematic effects which depend on the angle of observation and have to be checked on the basis of predetermined criteria or rules. It is not practically possible to use machines for reading information, for example images or numbers, encoded using this technique. Furthermore, the information content of these elements is very limited, and only an optical specialist will be capable of discriminating definitively between forgeries and an original.
Lastly, one should not ignore the fact that diffractive optical effects have in the course of time also been used outside the field of security, in particular for consumer articles such as wrapping paper, toys and the like, and the production methods for such elements have in the course of time become known to a large group of people and are correspondingly straightforward to imitate.
Further to the diffractive elements mentioned above, other components are also known which are suitable for optimum copy protection. These include optical components, as disclosed for example by EP-A 689,084 or EP-A 689,065, that is to say components with an anisotropic liquid-crystal layer, which latter has local structuring of the molecular orientation.
These components are based on a hybrid layer structure which consists of an orientation layer and a layer which is in contact with it and consists of liquid-crystal monomers cross-linked with one another. In this case, the orientation layer consists of a photo-orientable polymer network (PPN)xe2x80x94synonymous with LPP used in other literature which, in the oriented state, through a predetermined array, defines regions of alternating orientations. During the production of the liquid-crystal layer structure, the liquid-crystal monomers are zonally oriented through interaction with the PPN layer. This orientation which, in particular, is characterized by a spatially dependent variation of the direction of the optical axis, is fixed by a subsequent cross-linking step, after which a cross-linked, optically structured liquid crystal (LCP for liquid crystal polymer) with a preestablished orientation pattern is formed. Under observation without additional aids, both the orientation pattern itself and the information written into the liquid crystal before the liquid-crystal monomers are cross-linked, are at first invisible. The layers have a transparent appearance. If the substrate on which the layers are located transmits light, then the LCP orientation pattern or the information which has been written become visible if the optical element is placed between two polarizers. If the birefringent liquid-crystal layer is located on a reflecting layer, then the pattern, or the corresponding information, can be made visible using only a single polarizer which is held over the element. LCP authentication elements make it possible to store information, virtually without restriction, in the form of text, images, photographs and combinations thereof. In comparison with prior art authentication elements, the LCP elements are distinguished in that the authenticity of the security feature can be verified even by a layman since it is not first necessary to learn how to recognise complicated colour changes or kinematic effects. Since LCP authentication elements are very simple, reliable and quick to read, machine-readable as well as visual information can be combined in the same authentication element.
As is likewise already known, the complexity of LCP authentication elements can be increased further by inclining the optical axis of the LCP layer relative to the plane of the layer, uniformly or with local variation. This can be done in known fashion by producing a PPN layer with a locally varying tilt angle on the surface. This further provides a tilt effect, that is to say the information contained in the birefringent layer is seen with positive or negative contrast depending on the angle of observation. The object of the invention is now to provide further possible layer structures of the above-mentioned type for optical components, electro-optical devices and, in particular, for copy protection elements.
According to the invention, this is achieved in that the physical parameters and the configuration of the cross-linked liquid-crystal layer are varied and/or different layers, as well as a variety of substrates, with differing respective optical properties are combined. Since the layers which are used are generally transparent, they can also be applied successfully to already known, permanently visible authentication elements, for example watermarks, holograms or cinegrams. The retarder pattern of the liquid-crystal layer can then be seen in addition to the permanently visible authentication element on observation using a linear polarizer.
When using the transmissive birefringent layers described in EP-A 689,084, it is necessary to arrange one polarizer on each side of the element in order to read or make visible the information which is stored. A quick check of identity cards and the like is in this case made difficult by the involved positioning of the two polarizers above and below the authentication element. This disadvantage can be removed according to the invention by additionally integrating at least one polarization layer in the layer structure. If there is, for example, a polarization layer below the birefringent layer, then one external polarization sheet, held over the element, is sufficient for making the stored optical information visible.
A polarization layer integrated in the authentication element can, according to EP-A 689,084, be designed as a dichroic LCP layer. It is also possible to use a polarization sheet as a substrate for the PPN and LCP layers applied to it.
Where a reflector is present, which can be omitted according to this invention, the polariser sheet may possibly be the polariser for ingoing light and the analyser for outgoing light, which may not always be desirable.
A further disadvantage of the authentication elements described in EP-A 689,084 is that, when arranging a polarizer below the substrate, the polarization state of the light on passing through the substrate can be affected. If, for example, use is made of inexpensive polymer substrates which, by virtue of the way in which they are produced, are themselves birefringent then since the birefringence of these substrates is a random result of manufacture and varies from place to place, the birefringence of the LCP layer may, in the extreme case, be cancelled out, with the result that the information of the authentication element can no longer be read. Furthermore, the use of strongly scattering materials such as paper as a substrate is ruled out since polarized light would be immediately depolarized by these materials, so that the polarization state of the light which passes through and is analyzed using the second polarizer is unidentified and does not therefore carry any coded information.
However, if the integrated polarizer is, as proposed according to the invention, located between the substrate and the LCP layer, then the substrate has no effect on the polarization state of the light on passing through the LCP layer. As a result, on the one hand, it is possible to use inexpensive polymer substrates which, by virtue of the way in which they are produced, are themselves birefringent, and on the other hand the substrate need not be transparent. In this case, even scattering substrate materials can, for example paper and the like, are thus appropriate.
There are a variety of products, for example paintings, documents, photographs, compact discs, semiconductor chips, in which the authentication element need not be visible since this would impair the overall appearance of the product or would draw the attention of a potential product forger to the authentication element. For these cases, the invention proposes that orientable fluorescent dyes be incorporated in a transmissive structured LCP layer.
There are yet further optical effects which can be used for liquid-crystal authentication elements. Examples include those produced by cholesteric filters. A known feature of these filters is that they refract, with circular polarization, a fraction of the visible light spectrum in a wavelength range depending on physical parameters, while the unreflected light is transmitted (see: Schadt M., Fxc3xcnfschilling J., Jpn. J. Appl. Phys., 29 (1990) 1974). The effect of this is that the transmitted light and the reflected light have different colours. In order for this to produce visual effects, it is necessary for the wavelength range of the selective reflection to lie in the visible light range. For applications in which the information is read by machine, it is of course possible for the refraction band to lie outside the visible wavelength range.
Different types of optical components, which can likewise be used as authentication elements in the field of copy protection, are based on the combination of a linear polarizer, with a cholesteric filter. A configuration of this type makes it possible (as also further explained below) to produce different colours, for which use is in particular made of a second linear polarizer arranged on the opposite side of the cholesteric filter from the first polarizer.
Lastly, the tilt effect described at the start can also be produced in a different way than is already known. It is thus possible, according to the invention, to produce authentication elements whose tilt effects are more pronounced and whose production is even simpler from a technical point of view. This is achieved, in particular, in that at least one birefringent LCP layer of an element is constructed in such a way that its effective birefringence depends on the angle of observation. In this case, the optical axis may lie in the plane of the layer, i.e. it is not necessary to incur the extra cost of tilting the optical axis out of the plane in a defined way.
According to the present invention, there is provided an optical component comprising at least two layers, characterized by a retarder and a polarizer, the retarder having at least two regions with different optical axes. Preferably the retarder comprises an anisotropic layer comprising cross-linked liquid-crystal monomers. The retarder may be placed on an orientation layer and the orientation layer may be in contact with a polarizer. The orientation layer preferably comprises a photo-oriented polymer network (PPN). The polariser may be placed on a substrate. Optionally, a second polariser is arranged over the liquid-crystal layer and a further orientation layer and further liquid-crystal layer are arranged over this second polarizer, and the second liquid-crystal layer may also be structured. A further polariser may be arranged over the second liquid-crystal layer, and a third orientation layer and a third liquid-crystal layer are arranged over this further polarizer, and the third liquid-crystal layer may also be structured. An element for protection against forgery and/or copying may have an optical component as set forth above and an external linear or circular polarizer, the liquid-crystal layer encoding information which can be analyzed using the external polarizer. Such an element may be characterised in that the at least two liquid-crystal layers each encode a partial information content which together form a total information content. In this element, the liquid-crystal layer may be designed as a retarder and is preferably placed on a substrate characterized in that the substrate encodes a part of the total information content. Preferably, the external linear polarizer is structured, and both the liquid-crystal layer and the external polarizer each encode part of the total information content.
The optical component may be characterised by at least one circular polarizer, or preferably by two circular polarizers arranged one above the other, one of which rotates to the left and the other of which rotates to the right. An element for protection against forgery and/or copying may contain such an optical component and an external linear or circular polarizer for analysing the encoded information.
The invention also provides an optical component comprising an optically anisotropic layer which is formed by liquid-crystal molecules, characterised in that the optically anisotropic layer contains fluorescent molecules, and preferably has at least regions with different optical axes. The invention extends to an element for protection against forgery and/or copying including such an optical component.
The invention also provides an optical component comprising at least two layers, characterized by a cholesteric layer and a linear polarizer and preferably by an optically anisotropic layer, which may have regions with different optical axes. The optically anisotropic layer may be formed of cross-linked liquid crystal molecules. The cholesteric layer and the optically anisotropic layer are preferably on the same side of the linear polariser, which may be in contact with the cholesteric layer. The linear polariser may be arranged on a substrate, the cholesteric layer being in contact with the linear polariser, and an orientation layer may be placed on the cholesteric layer, and an optically anisotropic layer of cross-linked liquid-crystal monomers may be placed on the orientation layer, the liquid crystal (optically anisotropic) layer forming regions with different molecular orientations. An element for protection against forgery and/or copying have such an optical component and an external linear polarizer for analysing the information encoded in the liquid-crystal layer and/or in the cholesteric layer.
The invention also provides an optical component, containing a birefringent liquid-crystal layer which has at least two regions with different optical axes, characterized in that the optical delay of the liquid-crystal layer in the individual regions depends differently on the angle of observation. This component may be designed in such a way that the colour of the element on observation through a polarizer differs locally, and may be biaxial; preferably the birefringent layer is biaxial. An element for protection against forgery and/or copying may have such an optical component. A further element, according to the invention, for protection against forgery and/or copying comprises a polariser layer which has at least two regions with different polarisation directions.
A further such element is arranged on a substrate and comprises an optically anisotropic layer which has at least two regions with different optical axes, the substrate being a reflective polariser.
The invention also provides a device for protection against forgery and/or copying, wherein an element of any of the types set forth above and an analyser are arranged on the same substrate, such as a certificate or banknote.
Some of these may be considered as documents carrying invisible proof of authenticity, often in polarised light form. Some such documents, lacking a reflective layer, may be authenticable using illumination from underneath (transmitted through the document to the viewer). Some such documents may advantageously lack an integrated polariser.